In spiders, as we learn from Treviranus, the openventralspiracles of the scorpion are replaced by pseudo-spiracles;these inEpeira Diademaare three pair of small black points: on the back of the abdomen also arefourpair, but in some species there are onlytwo[2169]: the most remarkable, however, are exhibited by the cancriform spiders before noticed[2170]: inEpeira cancriformis, in the plate which covers the abdomen, they are dark red spots with an elevated rim and centre[2171]exactly resembling spiracles, except that they are not perforated; there are twenty-four of them, twenty arranged round the margin, and four in a square in the disk.
3.Organs of motion.In a former letter you were told that several insects are enabled to leap by means oforgansin their abdomen; I shall now describe such of them as require further elucidation. I then said thatPoduraandSminthurus, two apterous genera, take their leaps by means of an analfork[2172]. In the former genus the fork consists of a single piece attached to the under side of the anus, and terminating in a pair of long slender sharp processes which articulate with it and form the fork or saltatorious instrument[2173]. InSminthurusthetines, as they may be called, of the fork do not articulate with the base, but are of the same piece and consist of two joints, the terminal one being flat and obtuse[2174].Machilisto the anal fork adds eight pair of ventral linear springs (Elastes), which are covered with hair or scales, and terminate in a bristle or two. I have on a former occasion mentioned the natatorious laminæ with which the anusof the larva ofAgrionand of someDipterais furnished[2175]; the same part in that ofDytiscusends in a pair of tapering organs, fringed on each side like the hind-legs of the imago[2176], which doubtless assist it in swimming; those respiratory foliaceous laminæ which so singularly distinguish the abdomen of the larvæ ofEphemera, like the legs of the BranchiopodCrustacea, are probably used in some degree as fins, and aid their motions in the water[2177]. Under this head may also be mentioned the many-jointed bristles that form the long tails[2178]of the fly that proceeds from these larvæ, whose interesting history I long since enlarged upon; for when they fly the two lateral ones diverge from the central one, and perhaps perform the same office as the tail feathers (rectrices) of birds. These bristles are also to be found inMachilis[2179], and probably, as itsleapsare almost as long asflights, for a similar purpose, to steady their motion. I may here lastly state that I once saw aCryptophagus(CorticariaMarsh.), but I forgot to note the species, walking upon my window, which when it wanted to turn fixed itself to the glass by an inflated anal vesicle, and so accomplished its purpose.
4.Organs of Prehension[2180]. The abdominal organs ofprehensionare various; but as the great body of them are connected with the sexual intercourse of insects, I shall not consider them till I come to treat on that subject. The only remarkable one that is common to both sexes is that of the earwig, which is too well known to every child to call for any long description. The external organsofovipositionI shall also describe hereafter, and likewise those ofsecretionthat have not already been noticed.
5.Weapons.As thestingsof someHymenopteraare analogous to theovipositorsof the majority of that Order, I shall consider them both together when I treat of the sexual organs of insects; but there is one, and that a tremendous one, not connected with those organs, which may be noticed here. I mean the sting of thescorpion. There appears to be some analogy between the poisonous fangs of one tribe of the Ophidian reptiles[2181], the mandibulæ of spiders[2182], the second pair of pedipalps, or the fangs of theScolopendridæ[2183], and the organ in question[2184]; but the last possesses this peculiarity, that it is placed at the opposite extremity of the body, where it is preceded by a long jointed tail, which properly speaking is merely a continuation of the abdomen, since the spinal marrow, the intestinal canal, and thepseudocardia, are extended into it[2185]. Providence might have a double view in thus contracting the dimensions of this part of the abdomen; in the first place, the animal is by this enabled to turn its tail over its back preparatory to its inflicting a wound, and in the second, perhaps, this formation favours the sublimation of the venom, the long tail acting as an alembic for that purpose. This machine consists of six angular joints including the sting, the last but one being the longest, and the last inflated, as it were, at the base, and terminating in a sharp subulato-conical point which curves downwards,and has an orifice in a channel at the end on each side. Treviranus could not discover these orifices in the sting ofScorpio europæus[2186]; they may however be readily seen if viewed with a sufficiently high power, though not under a common pocket microscope. Whether the very slender, many-jointed,realtail of the remarkable genusThelyphonusis used in any respect as aweapon, has not been ascertained: it is a filiform hairy organ consisting in some specimens of more thantwentyjoints, the first being very much larger than the rest[2187].
6.Appendages[2188]. We are lastly to advert to thoseappendagesof the abdomen of which the use is not at present discovered. These are the styles (styli) of theStaphylinidæ; the leaflets (foliola) of theLibellulina; the floret (flosculus) of theFulgoræ; thecerciof theBlattidæandGryllina; and the threads (fila) ofMachilis: but having nothing important to add concerning them, the definitions of those terms will give you a sufficiently clear idea of them[2189]. As they are common to both sexes, if their use is connected with the sexual intercourse, it must be similar to that which Treviranus ascribes to the pectens of scorpions, they must be instruments of excitement.
And now, after this long discourse on the External Anatomy and structure of these little beings, you may think perhaps at first that the subject is exhausted; andthat I must have discovered and described every part and every variation of every part of the crust of an insect. But when you go on to reflect what a comparatively small number of these creatures have fallen under my examination, and in those, after all my laborious and painful researches, from my limited faculties and other imperfections of our common nature, how much will probably have eluded my notice, you may conclude that thousands of facts still remain concealed to reward the patient assiduity of future investigators. Such are the immensity and variety of the works of theCreatorin this department, that it would require a long life, and fill volumes upon volumes, to discover and give a description of all the peculiarities of structure of the insects that are already known; and could all that exist[2190]be so studied and explained in full detail, the library that the Calif Omar ordered to be burned at Alexandria could scarcely have contained more books than would be required to receive the results. But "who is sufficient for these things[2191]?" This is a question that the most able and most experienced physiologist must often feel disposed to put to himself when, lost in the intricate labyrinth of the works of his Maker, he sees all things arranged, "wheel within wheel," in an order that he can onlypartiallyunravel, instead of tracing the "regular confusion" through all its windings. But glimpses of light, and points of irradiation, here and there discover tofragmentsof the truth of things, and such vestigesof the grand system of theDeity, as enable him in some degree to appreciate, and dispose him humbly to adore thatWisdom,Power, andGoodness, that at first created and now sustains in its full beauty and harmony thewondrous whole.
I am, &c.
END OF THE THIRD VOLUME.
Printed by Richard Taylor,Shoe-Lane, London.
Plate VIPlate VI
Plate VI
PLATE VI.[2193]
Plate VIIPlate VII
Plate VII
PLATE VII.[2194]
Plate VIIIPlate VIII
Plate VIII
PLATE VIII.[2196]
Plate IXPlate IX
Plate IX
PLATE IX.[2197]
Plate XPlate X
Plate X
PLATE. X.[2198]
N.B. In this plate theredpoints out thecostal, and theyellowtheanalareas, theintermediatebeinguncoloured.
N.B. In this plate theredpoints out thecostal, and theyellowtheanalareas, theintermediatebeinguncoloured.
Plate XIPlate XI
Plate XI
PLATE XI.[2199]Antennæ.
Plate XIIPlate XII
Plate XII
PLATE XII.Antennæ.
Plate XIIIPlate XIII
Plate XIII
PLATE XIII.[2200]
Plate XIVPlate XIV
Plate XIV
PLATE XIV.[2201]
Plate XVPlate XV
Plate XV
PLATE XV.[2202]
Plate XVIPlate XVI
Plate XVI
PLATE XVI.[2203]
Plate XVIIPlate XVII
Plate XVII
PLATE XVII.[2204]
Plate XVIIIPlate XVIII
Plate XVIII
PLATE XVIII.[2205]
Plate XIXPlate XIX
Plate XIX
PLATE XIX.[2206]